A wide variety of radically polymerizable monomers, such as methacrylic and acrylic monomers, is commercially available and can confer a wide range of properties to a polymer or copolymer (hereinafter, collectively referred to as (co)polymer). The use of conventional free radical (co)polymerization methods to synthesize (co)polymers provides little control over molecular weight, molecular weight distribution and, in particular, (co)polymer chain structure.
In order to overcome this problem, polymerization methods based ionic methods (anionic and cationic polymerization) were developed that would enable the artisan a degree of control over the placement of monomers along a growing polymer chain. These methods are limited, however, to a relatively narrow class of monomer and polymer types.
A further development provides a method of free radical polymerization, in which a “living” polymer containing a radically transferable atom or group is employed to enable a degree of control over (co)polymer composition and architecture. These methods, referred to collectively as atom or group radical transfer polymerization (ATRP), are described in, for example, U.S. Pat. Nos. 5,807,937, 5,789,487 and 5,763,548 to Matyjaszewski et al. The ATRP method is described as providing highly uniform products having controlled structure (i.e., controllable topology, composition, etc.).
However, ATRP type processes require the use of halogenated hydrocarbon initiators and transition metal catalysts, which cause safety and material compatibility concerns when implemented at production scale. Further, although more versatile than ionic polymerization methods, a number of functional monomers, for example carboxylic acid functional monomers, cannot be polymerized directly using ATRP methodologies.
A further technique that has been explored to provide control over a radical polymerization process is those processes utilizing iniferter initiators. Iniferter initiators contain a chemical bond that will break under appropriate thermal or photolytic conditions, forming two carbon centered radicals. The radicals are capable of polymerizing monomers and the polymerization step competes with radical recombination. Functional groups that have found use in iniferter initiators include thiuram disulfides, dithiocarbamate disulfides and ethylene derivatives that, contain at least four stabilizing groups. Thiouram iniferter initiators are disclosed in, for example, U.S. Pat. No. 6,169,147 to Kroeze et al.; Lokaj et al., Journal of Applied Polymer Science, 67, 755-762 (1998); Kroeze et al., Macromolecules, 28, 6650-6656 (1995); Nair et al., J. Macromol. Sci.-Chem., A27(6), 791-806 (1990); and Nair et al., Polymer, 29, 1909-1917 (1988). Thiuram disulfide iniferter initiated polymerizations are able to produce block copolymers, but typically with limited control and wide molecular weight distributions.
Dithiocarbamate disulfide iniferter initiators are disclosed in, for example, U.S. Pat. Nos. 5,866,047 to Nagino et al., 5,658,986 to Clouet and 5,489,654 to Clouet; Suwier at al., Journal of Polymer Science: Part A: Polymer Chemistry, 38, 3558-3568 (2000); and Nair et al., Macromolecules, 23, 1361-1369 (1990).). Dithiocarbamate disulfide iniferter initiated polymerizations are able to produce block copolymers, but typically with limited control and wide molecular weight distributions.
Various multisubstituted ethylene derivatives have been disclosed as iniferter initiators. For example, U.S. Pat. No. 5,866,047 to Nagino et al., Chen et al., European Polymer Journal, 36, 1547-1554 (2000), Tharanikkarusa et al., Journal of Applied Polymer Science, 66, 1551-1560 (1997); and Tharanikkarusa et al., J.M.S.-Pure Appl. Chem., A33(4), 417-437 (1996) disclose derivatives of 1,1,2,2-tetraphenyl-1,2-ethanediol as iniferter initiators. The use of phenylazotriphenyl methane as an iniferter initiator is disclosed by Otsu et al., Polymer Bulletin, 16, 277-284 (1996). Various 1,2-dicyano-1,2-diphenylethane derivatives are described as iniferter initiators by Qin et al., Macromolecules, 33, 6987-6992 (20.00); Qin et al., Journal of Polymer Science: Part A: Polymer Chemistry, 38, 2115-2120 (2000); Qin et al., Polymer, 41, 7347-7353 (2000); Qin et al., Journal of Polymer Science: Part A: Polymer Chemistry, 37, 4610-4615 (1999); Tharanikkarusa et al., European Polymer Journal, 33, 17,79-1789 (1997); Tazaki et al., Polymer Bulletin, 17, 127-134 (1987); and Otsu et al., Polymer Bulletin, 17, 323-330 (1987). The multisubstituted ethylene derivative iniferter initiated polymerizations are all able to produce block copolymers, but typically also demonstrate limited control of the polymerization process and the resulting polymers had wide molecular weight distributions.
All of the cited iniferter technology utilized either disulfide groups or ethylene derivatives with multiple radical stabilizing groups, such as, phenyl and cyano, to effect a somewhat controlled polymerization process. In the case of disulfide iniferters, the sulfide and dithiocarbamate groups are prone to side reactions and early termination reactions. The multi-phenyl substituted ethylene iniferters are similarly prone to side reactions and early termination reactions. These problems lead to poor control of the polymerization process and a wide, bimodal or multimodal-molecular weight distribution.
There remains a need for a method of controlled polymerization, which is capable of polymerizing a wide variety of functional monomers. The controlled polymerization method should also provide for copolymer composition and architecture control as well as providing for control over polymer molecular weight and molecular weight distribution.